The spine is formed of a column of vertebra that extends between the cranium and pelvis. The three major sections of the spine are known as the cervical, thoracic and lumbar regions. There are 7 cervical vertebrae (C1-C7), 12 thoracic vertebrae (T1-T12), and 5 lumbar vertebrae (L1-L5), with each of the 24 vertebrae being separated from each other by an intervertebral disc. A series of about 9 fused vertebrae extend from the lumbar region of the spine and make up the sacral and coccygeal regions of the vertebral column. The natural curvature of the spine includes a combination of lordosis and kyphosis. Specifically, the cervical and lumbar portions of the spine exhibit a natural lordotic curvature, meaning that they are set in a curve that is anteriorly convex (and posteriorly concave). The thoracic portion of the spine has a naturally kyphotic curvature, meaning that it is set in a curve that is anteriorly concave (and posteriorly convex).
The main functions of the spine are to provide skeletal support and protect the spinal cord. Even slight disruptions to either the intervertebral discs or vertebrae can result in serious discomfort as well as compression of nerve fibers either within the spinal cord or extending from the spinal cord. If a disruption to the spine becomes severe enough, severe pain, disability and damage to a nerve or part of the spinal cord may occur and can result in partial to total loss of bodily functions (e.g., walking, talking, breathing, etc.). Therefore, it is of great interest and concern to be able to both correct and prevent any ailments of the spine.
Fixation systems are often surgically implanted to stabilize or immobilize a portion of the spine. They are generally utilized during spinal fusion procedures to immobilize the applicable vertebrae until bone growth occurs to effect the fusion and/or to correct vertebral alignment issues. Fixation systems often use a combination of rods, plates, pedicle screws, and bone hooks to attach a fixation construct to the affected vertebrae. The configuration required for each procedure and patient varies due to the ailment being treated, the specific method of treatment (e.g. surgical approach, etc. . . . ) and the patient's specific anatomical characteristics.
Depending upon the pathology presented, correction of spinal ailments may involve only one vertebral level (i.e. a single intervertebral disc and the two vertebral bodies adjacent that intervertebral disc) or multiple spinal levels. An extreme example of a multiple level treatment relates to deformity correction (e.g. scoliosis correction) in which a screw and rod construct is implanted along a significant length of the spine in an attempt to forcibly correct or maintain a desired spinal alignment.
Whatever the treatment, the goal remains to improve the quality of life for the patient. In the vast majority of cases this goal is achieved, however in some instances patients who receive implants to treat the primary pathology develop a secondary condition called junctional disease. Most commonly this occurs at the proximal or cephalad area of spinal instrumentation and is then termed adjacent segment pathology. Clinical Adjacent Segment Pathology (CASP) refers to clinical symptoms and signs related to adjacent segment pathology. Radiographic Adjacent Segment Pathology (RASP) refers to radiographic changes that occur at the adjacent segment. A subcategory of CASP and RASP that occurs at the proximal end of the instrumentation is termed proximal junctional kyphosis (PJK). PJK may be defined in several manners and commonly is specified as kyphosis measured from one segment cephalad to the upper end instrumented vertebra to the proximal instrumented vertebra with abnormal value defined as 10 degrees or greater. In practice this often means that the patient's head and/or shoulders tend to fall forward to a greater degree than should normally occur. Sometimes the degree is significant.
Adjacent segment pathology can occur as either a degenerative, traumatic or catastrophic condition and sometimes as a result from a combination of factors. Degenerative conditions are ones that occur over a period of time, normally 5 or 6 years but can occur at an accelerated rate particularly with altered mechanics related to spinal fusion. As a result the patient's head and/or shoulder region(s) fall forward gradually over time. Traumatic and catastrophic conditions occur as a generally sudden shifting of the vertebral body immediately cephalad to the upper end instrumented vertebra and can lead to sudden changes in spinal alignment with marked symptoms noted by the patient.
Whether the condition is degenerative, traumatic or catastrophic, the exact cause of adjacent segment pathology is uncertain. Generally, it is believed that adjacent segment pathology and more specifically PJK is a result of excess strain and stress on the proximal instrumented spinal segment which is then at least partially transferred to the bone structures, disc, ligaments and other soft tissues, causing a loss of normal structural integrity and mechanical properties. The resultant effect can be a forward (i.e. kyphotic) shift of the adjacent non-instrumented vertebral body. One such theory is that this strain and stress is caused by suboptimal alignment and/or balance of the screw and rod construct. Another theory is that the rigidity of the screw and rod construct causes the problem in that the transition from a motion-restrained segment to a motion-unrestrained segment is too much for the non-instrumented (unrestrained) segment to handle over time. Yet another theory speculates that the specific level at which the proximal instrumented vertebra is located is of vital importance in that some levels may be better suited to handle a proximal termination of a fixation construct than others.
Thus there remains a need for continued improvements and new systems for spinal fixation with a specific goal of preventing the occurrence of or reducing the degree of adjacent segment pathology and failures occurring at either the distal junction (DJK) or proximal junction (PJK). The implants and techniques described herein are directed towards overcoming these challenges and others associated with posterior spinal fixation.